Circuit breakers and cut-outs are already known which have what is known as a short-circuit indicator. This is the case particularly with cut-outs having magneto-thermal trips provided with a device indicating the magnetic or thermal mode of operation of the trip. Such a device is described for example in French Pat. No. 1 401 630.
For the user it is indeed of interest to know whether the tripping of the circuit breaker is a result of magnetic or thermal operation. But in many cases indication of the magnetic functioning of the trip will be inadequate as a means of judging whether there is an actual short-circuit or simply an abnormally high over-current. Furthermore, the structural elements of such an indicator are directly linked with those of the trip, which considerably limits the possibilities of construction.
The progress at which the present invention aims resides in providing circuit breakers with a short-circuit indicator, the functioning of which is linked not to the tripping of the protective devices but to the power developed in the breaking chambers of the apparatus and more precisely the wave of gas pressure which results therefrom.
Air-type circuit breakers intended to break short-circuited currents generally comprise a fixed contact and a moving contact per pole disposed inside a relatively closed breaking chamber. In the event of over-voltage, under the action of protective trips or the direct effect of electro-dynamic stresses generated by the current, the contacts open and an electric breaking arc appears, the voltage of which depends upon the structural characteristics of the breaking chamber and the current intensity. The higher the current intensity, the greater will be the power of the electric breaking arc. The effect of the power of the arc gives rise to a gas pressure wave in the breaking chamber resulting from the sudden rise in temperature and possibly from the emission of gases through the walls of the breaking chamber. The amplitude of this pressure wave is linked to the power of the breaking arc and thus to the intensity of the current actually broken by the apparatus. For low levels of current, the pressure will be small and in contrast for high currents such as those which may result from a short-circuit the pressure may attain levels sufficient to actuate a pressure-sensitive indicating device.